Water and oil relative permeabilities of reservoir rocks are routinely needed in the analysis of a waterflood and enhanced oil recovery projects. Such data are, for example, required in almost all flow and recovery calculations. Among various ways to measure relative permeabilities, the so-called steady-state method and transient method are the two most common techniques. The transient method (also known as displacement or "Welge" method) is easy and efficient. However, useful data are obtained only when the displacing fluid is more mobile than the displaced fluid. Therefore, imbibition relative permeabilities of a water-wet rock can be generated with water displacing oil but good drainage curves cannot be obtained with oil displacing water. Furthermore, displacement tests with an unfavorable mobility ratio may be plagued by viscous fingering problems.
By contrast, the steady-state method can be used to obtain accurate relative permeabilities for either oil wet or water wet rocks. In the steady-state method, water and oil are injected simultaneously in known relative amounts into a core. Pressure differential across the core during flow is measured. After fluid saturations within the test core have reached equilibrium, the saturations are determined by one of many techniques such as gravimetrical, electrical resistivity, X-ray or microwave absorption. One simple technique to determine fluid saturations involves recycling of the water and oil from the core into a holding tank. By noting the rise and fall of the water-oil interface inside the holding tank, changes of fluid saturations inside the test core may be calculated by a simple material balance.
The experimental apparatus and procedures for steady-state relative permeability measurements are thus straightforward. There is, however, a major drawback. The steady-state method is time-consuming and tedious. It may take quite a while to reach a steady state at each pump setting, especially in the neighborhood of residual saturations. Meanwhile, an operator's attention is required to record the data and to determine if an equilibrium has been attained. For this reason, the steady-state method is not as widely used as the transient method for measuring two-phase relative permeabilities although it is the surest way to obtain correct results. To date, however, there has not been a satisfactory method of automating the system.
Early attempts at making automated relative permeability measurements have resulted in complex and, often, unworkable devices. For example, one automated system recently developed for relative permeability measurement was based on a centrifuge technique (D. S. O'Meara, Jr., et al., SPE Paper 12128, "Multiphase Relative Permeability Measurements Using an Automated Centrifuge", 1983). It has been found that this method is unsatisfactory because it requires spinning down the oil-water mixture in the core while "pictures" are taken of the spinning core. This can be time consuming, complex and, therefore, prone to failure. It is clear that an improved automated relative permeability measuring system is needed.